Adds support for (renamed) packages that begin with numbers.

Previously, goimports had a caveat where it only supported package that were named after the base of their import path. Given that, a reasonable optimization was to skip all folders that begin with a number, because Go packages may not begin with a number, so such folders could not have been supported packages.

Now that the caveat is removed, it's possible to have an import path, e.g., `github.com/something/123` and inside the folder `123`, there may be a valid package named `go123`.

This fix allows goimports to properly support such packages.